The present invention relates to a cement composition for laying underwater wherein a cement constituent thereof is not separated or washed away in water.
As concrete (containing mortar) is being laid under water (containing sea water and mud), the cement may become separated or washed away from a part of the aggregate or subjected to stirring effect upon contact with the water, either as the concrete mix is falling through the water or as the concrete mix is spreading in a widthwise direction under water. The part of the concrete mix upon contact with the water is reduced in strength, resulting in many defects as a concrete mix when laid under water.
As a concrete laying operation is performed under water (sea or river), the surrounding area is contaminated. Besides, a fishery compensation problem occurs and makes the concrete laying operation difficult to perform.
In order to prevent the cement from becoming separated from the concrete mix in water, a viscosity building agent or viscosity builder such as a polyacrylamide compound or the like may be mixed with an aggregate in an amount ranging from about 2% to about 5% by weight based on the cement content of the mix (referring to FIG. 1).
TABLE 1 ______________________________________ Compressive Ratio of strength of strength Amount of specimen at With ture of admix- 28th day Admixture Chemical ture per Separa- (kg/cm.sup.2) (under water) component 100 kg of tion of Laid Without ad- of admix- cement cement Laid under mixture (in ture (g) (%) in air water air) ______________________________________ -- 0 21.6 438 137 0.32 Poly- 2,000 6.8 173 103 0.24 acrylo- nitrile Poly- 3,000 15.7 98 37 0.08 metha- crylate Poly- 5,000 15.9 120 37 0.08 acrylic ester ______________________________________
However, when this method is adopted, if the unit amount of cement is 400 kg/m.sup.3, for example, the amount of viscosity builder to be used is 8 to 20 kg per 1 m.sup.3 of the concrete mix. This means that the cost of the viscosity builder is equal to or more than the concrete mix itself. If the amount of the viscosity builder increases and the use of it becomes costly, the improvement in the performance of the viscosity builder by addition of such a viscosity builder does not allow actual adoption of this method.
Moreover, a concrete mix containing such a polymer viscosity builder tends to have a delayed setting time and to have a lower strength than a concrete mix containing no such agent. This drawback is the biggest obstacle in improving the performance of concrete laid under water by the use of a viscosity builder.